The present invention relates to a current transformer in which the current in a primary conductor induces a current in a second conductor.
Conventional AC current transformers fall into two categories; magnetic core current transformers such as the commonly used iron-core toroidal type, in which the primary conductor passes through the centre of the toroid so as to induce via the magnetic core a current in the secondary conductor coiled around the toroid, and air coupled current transformers in which a central coil defining the secondary coil is wound onto a non-magnetic material and the flux generated by a primary conductor is coupled to the secondary (at least in part) via an "air gap". A typical design for such transformers is shown in U.S. Pat. No. 4,473,810 (Souques) in which the secondary coil lies within the loop of a U shaped primary conductor, the whole assembly being surrounded by a shielding in the form of a box of ferromagnetic material. The shielding also serves as a path for the flux generated by the primary conductor.
Iron core transformers suffer from the disadvantage that the presence of DC components in the primary conductor cause saturation of the magnetic core and render the transformer insensitive to AC signals. Air coupled current transformers are immune to such saturation but suffer from the disadvantage that they are susceptible to the effects of external magnetic fields and normally require a magnetic shield made of a suitable nickel-iron alloy, such as mumetal, surrounding the sensor. Air coupled current transformers futher suffer from the disadvantage that they are relatively insensitive to low currents in view of the lower permeability of air as compared to a ferromagnetic material. Increasing the sensitivity and scale factor is conventionally achieved by increasing the number of turns in the coils, which leads to problems where a compact design of sensor is required.
A modified air coupled current transformer using a ferromagnetic element within the centre of the secondary coil is disclosed in DE 0 481 104. The magnetic circuit through which flux flows is defined by the ferromagnetic element, a ferromagnetic shield and the air gap between the element and shield. As with conventional air coupled current transformers, the position of the element and core within the sensor and, in particular, in relation to the surrounding shield, is critical in determining the scale factor or relative magnitude of the primary current to the secondary current. This sensitivity to the positioning of the coil is a further disadvantage of air coupled current transformers as it can be difficult to ensure regularity of performance in a batch of produced transformers.